A porous material, which is a structure containing pores, is widely used as an industrial material utilizing its porous structure, such as a filter, a fluid permeable member, a catalyst carrier, an adsorbing material, or an insulating material. Especially, a porous material having unidirectional through-pores is widely used as a filter, a fluid permeable member, or the like. In the development of such applications, the permeation of a large volume of fluid, the increase of trapping efficiency, or the upsizing of the member represents a challenge.
As a method for producing a porous material having unidirectional through-pores, there is proposed a method using the phase separation of a raw material, particularly involving freezing water contained in the raw material followed by removing ice by drying, that is, a method using the ice as a pore source. This method is, for example, a method which involves mixing water and a particulate material as a raw material to make a slurry, pouring the slurry into a die, freezing the slurry from the bottom of the die to grow ice, and removing the ice from portions where the ice is formed by drying to provide pores.
Examples of the method include the following propositions which have previously been made.
For example, a method for producing a ceramic molded product having macropores has been proposed, which involves preparing a slurry in which a ceramic powder as a raw material is dispersed in water, freezing the resultant slurry from a specific direction to promote the growth of ice, and subjecting the frozen slurry to vacuum-freeze drying to sublime the ice to provide the ceramic molded product having macropores (Patent Literature 1).
Meanwhile, a method for producing a resin porous material has been proposed, which involves forming a three-dimensional network structure between a polymer of a water-soluble organic monomer and a water-swellable clay mineral exfoliated in layers and removing the contained solvent by freeze-drying to provide the resin porous material (Patent Literature 2).
A method for producing a porous biomaterial has also been proposed, which involves mixing hydroxyapatite/collagen composite fiber and buffer solution, growing ice crystals by freezing treatment, and then providing unidirectional pores by drying (Patent Literature 3).
In addition, mention is made of a method for producing a porous ceramic material having unidirectional pores, which involves freezing an aqueous solution of a ceramic precursor, freeze-growing water of the resultant unidirectionally in the form of a column, and then dry-removing the ice (Patent Literature 4).
The present inventors have also proposed a new method for molding a ceramic porous material, in which gelation and freezing are combined (Patent Literature 5).
According to the method, a step of gelation is added to improve moldability as compared to existing techniques and to suppress the growth of dendritic ice crystals. The gelling agent retains water, enabling the provision of an extremely high porosity.
In the above freezing methods, that is, the methods which each involve freezing a mixture of a raw material and water from a specific portion thereof, one portion contacting a refrigerant has a low temperature and another portion at a greater distance from the refrigerant has a higher temperature; thus, the frozen material has a non-uniform temperature distribution. Generally, ice crystals are finer when formed at a lower temperature and coarse ice crystals are formed at high temperature (Non Patent Literature 1).
Thus, a problem was that ice in a portion contacting a refrigerant has a fine size whereas ice at a greater distance from the refrigerant forms coarse ice crystals. In addition, there was a problem that a greater distance from a refrigerant increases the ice formation temperature since latent heat with freezing is released in the formation of ice.
Another conventional problem was that it is difficult to suppress re-crystallization of ice occurring within a frozen material during the formation of a porous material. The re-crystallization of ice means re-crystallization after the dissolution of the whole or part of ice crystals. More simply, it means the sticking of ices to each other. For example, the freezing of an aqueous material having a particularly high water content such as a gel or an ice cream easily leads the re-crystallization of ice in the resultant. When unidirectional freezing is slowly performed, a bundle of ices is easily formed since there is sufficient time for the re-crystallization of ice to occur. The re-crystallization of ice cannot easily be suppressed even by close attention to the composition of an aqueous material, the freezing temperature, and the like, which has been responsible for the hindrance of the execution of the technique.
As described above, the freezing method is an excellent method in that it can provide unidirectionally oriented pores, but has had a problem that it causes the size or thickness of an ice formed in a raw material or a gel to be non-uniform.